With a sound-absorbing element of this type, as described in its basic design in German Offenlegungsschrift No. 2,758,041, sound is absorbed by natural vibrations of the surfaces of the cup-shaped recesses, mainly vibration of the bottom surfaces, the dimensions of which bottom surfaces are selected so that their natural vibrations fall within the frequency range of audible sound.
The two essential characteristics of such sound-absorbing elements are the relative distribution of the acoustic absorptivity over the different acoustic frequencies and the absolute acoustic absorptivity for the different acoustic frequencies over the complete acoustic frequency range. The relative distribution of the acoustic absorptivity should be distributed as evenly as possible over the complete acoustic frequency range, bearing in mind the frequency-dependent acoustic sensitivity of the human ear and the acoustic frequency spectrum which occurs in each case at the place of application of the sound-absorbing element, so that the sound energy which arises is absorbed as evenly as possible over the complete acoustic frequency spectrum. The absolute acoustic absorptivity for the different acoustic frequencies should be as high as possible, so that as much sound energy as possible is absorbed and so that the sound level is reduced as much as possible. The two above-mentioned characteristics may be represented by a so-called sound absorption curve which reproduces the dependence of the acoustic absorptivity on the acoustic frequency.
Thus, it is desirable to achieve a high integral acoustic absorptivity, which may be ascertained by integrating the sound absorption curve over the acoustic frequency range which is of interest, with as even a distribution of the acoustic absorptivity as possible over the different acoustic frequencies. As described in German Offenlegungsschrift Nos. 2,758,041 and 2,921,050, the combined disclosures of which correspond to U.S. Pat. No. 4,425,981, the relative distribution of the acoustic absorptivity over the different acoustic frequencies may be evened out by making the number of the possible different natural vibrations and of the harmonics thereof and of the harmonic oscillations of these natural vibrations in general as large as possible. This may be effected in a variety of ways, for example by designing the bottom surfaces of the cup-shaped recesses to be rectangular instead of square, because rectangular plates have more natural vibration modes than square plates, and by providing several groups of cup-shaped recesses which are adjacent in a grid-shape and which are distinguished in that the bottom surfaces of the various groups have different sizes.
Thus, it has been shown from the experiments which have led to the present invention that with khown sound-absorbing elements, in which the cup-shaped recesses have in each case a bottom surface of 8.times.9 cm, it is not possible to absorb the sound energy in a satisfactory manner simultaneously at low and high acoustic frequency ranges. This fact is explained later on with reference to FIG. 6.
An absorption improvement may be achieved theoretically and practically in the higher acoustic frequency range by using cup-shaped recesses which have smaller bottom surfaces, for example with dimensions of 9.times.4 cm, but at the same time, a deterioration results in the acoustic absorptivity in the middle and low acoustic frequency ranges. These conditions will be explained later on using FIG. 7. On the other hand, if cup-shaped recesses are used which have larger bottom surfaces, then conversely, an improvement is achieved in the acoustic absorptivity in the low acoustic frequency range, but in this case the acoustic absorptivity in the middle and higher acoustic frequency ranges simultaneously deteriorates.
Therefore, in order to even out the acoustic absorptivity for the different acoustic frequencies, it would be necessary, as already indicated above and as described in German Offenlegungsschrift No. 2,921,050 to provide adjacent cup-shaped recesses having large and small bottom surfaces in the sound-absorbing element. However, a disadvantage of such a solution is that the absolute acoustic absorptivity falls, because, apart from overlaps in the middle acoustic frequency range, only one half of the cup-shaped recesses are effective for the lower acoustic frequency range and only the other half of the cup-shaped recesses are effective in the upper acoustic frequency range, if one proceeds from the fact, for example that half the total number of cup-shaped recesses are composed of those having comparatively small bottom surfaces and the other half are composed of those having comparatively large bottom surfaces.
Thus, although the absorption curve is evened out for a given total surface of the bottom surfaces of the cup-shaped recesses, i.e. an improvement in the relative distribution of the acoustic absorptivity, it is over a much flatter level of the absolute acoustic absorptivity for the different acoustic frequencies, so that the absolute integral acoustic absorptivity for the complete acoustic frequency range is not improved, as is to be expected.